The U.S. European command says the Russian planes buzzed a U.S. ship multiple times Monday and Tuesday in what the U.S. officials described as an unsafe and unprofessional manner.

Secretary of State John Kerry told CNN Espanol that under U.S. military rules of engagement, the Navy ship could have opened fire. He condemned the Russian actions.

Maj. Gen. Igor Konashenkov, spokesman for the Russian defense ministry, said Thursday that the pilots of Russian Su-24 jets saw the ship and turned back “while using all measures of precaution.” Konashenkov said he was baffled by what he described as the “distressed reaction of our American counterparts.”

The U.S. European Command statement on Wednesday said officials are using diplomatic channels to address the matter.

“It may have been in international waters and therefore may not be any violation of international law, but it is still irresponsible behavior,” said NATO Deputy Secretary General Alexander Vershbow, a former US ambassador to Russia.

“It illustrates that Russia is contributing to tensions,” he said, speaking on the sidelines of a security conference in Kiev, the Ukrainian capital.